Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) imagers, are commonly used in photo-imaging applications.
A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate, photoconductor or photodiode having a sensing node, typically a floating diffusion node, connected to the gate of an output transistor. The imager may include at least one electronic device such as a transistor for transferring charge from the underlying portion of the substrate to the floating diffusion node and one device, also typically a transistor, for resetting the node to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate. For photodiodes, image lag can be eliminated by completely depleting the photodiode upon readout.
CMOS imagers of the type discussed above are generally known as discussed, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524 and 6,333,205, assigned to Micron Technology, Inc., which are hereby incorporated by reference in their entirety.
FIG. 1 illustrates a block diagram for a CMOS imager 10. The imager 10 includes a pixel array 20. The pixel array 20 comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array 20 are all turned on at the same time by a row select line and the pixels of each column are selectively output by a column select line. A plurality of rows and column lines are provided for the entire array 20.
The row lines are selectively activated by the row driver 32 in response to row address decoder 30 and the column select lines are selectively activated by the column driver 36 in response to column address decoder 34. Thus, a row and column address is provided for each pixel. The CMOS imager 10 is operated by the control circuit 40, which controls address decoders 30, 34 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 32, 36, which apply driving voltage to the drive transistors of the selected row and column lines.
The pixel signal output from the pixel array is analog voltage. This pixel output signal must be converted from an analog signal to a digital signal. Thus, the pixel output signal is usually sent to an analog-to-digital converter (ADC) (not shown in FIG. 1). Many CMOS image sensors use a ramp analog-to-digital converter, which is essentially a comparator and associated control logic. In the conventional ramp analog-to-digital converter, an input voltage of the signal to be converted is compared with a gradually increasing reference voltage. The gradually increasing reference voltage is generated by a digital-to-analog converter (“DAC”) as it sequences through and converts digital codes into analog voltages. This gradually increasing reference voltage is known as the ramp voltage. In operation, when the ramp voltage reaches the value of the input voltage, the comparator generates a signal that latches the digital code of the DAC. The latched digital code is used as the output of the analog-to-digital converter.
Unfortunately, variation in the power supply, noise and precision of the reference voltage adversely impacts the performance of the analog-to-digital converters used in today's image sensors. Accordingly, there is a need and desire for an improved analog-to-digital converter for image sensors. There is also a need and desire to reduce the amount of power consumed during the operation of the image sensor.